Would the transfer rate be approx 98 MB/sec ? No idea what details you gave your vendor, but chances are it might be an [optimistic} estimate., since they may know their performance specs best. Alot of variables to consider, including who or what else might be using the NAS and how well it is interfaces to the server's storage subsys, etc. I'd suggest making a test db of know size, and do some basic sanity-checks, then project with alot leeway ! Best luck in any event.

This will depend on your IO speed, network bandwidth of source server, network at destination, IO at destination, the amount of work the NAS is doing at any time during the backup from other tasks, etc etc etc

I've also seen weird speed differences between backup times when backing up to the same destination which I could only explain by differences in data and index structure

You should also look at upgrading to 2016 or 2017 or moving backups to something like Commvault which does pretty awesome compression on SQL. BAK files are very inefficient